A display device including a transmissive or transflective display panel (e.g., a transmissive or transflective liquid crystal display panel) usually includes an illuminating device (referred to also as a “backlight unit”) disposed behind the display panel. Light emitted from the illuminating device is projected onto a back face of the display panel, whereby an image is displayed visible on a front face of the display panel.
A side light type illuminating device (referred to also as a “side light type backlight unit” or an “edge light type backlight unit”) is sometimes used in the display device. A conventionally-used general side light type illuminating device includes a light source board that defines a linear light source, and a light guide plate arranged to convert light emitted from the light source board into planar light (to convert the light source board into a planar light source). Then, the light emitted from the light source board enters the inside of the light guide plate from a face on a lateral side (an end face of a peripheral portion in a plane direction) of the light guide plate, and is emitted to the outside from a front face of the light guide plate. The light emitted from the front face of the light guide plate is projected onto a back face of the display panel.
It is preferable that the light projected onto the back face of the display panel has uniform intensity distribution in its plane direction in order to prevent luminance unevenness from developing in the image that the display device displays. Thus, it is necessary to uniformalize intensity distribution of the light that enters the inside of the light guide plate.
The light source board used in the side light type illuminating device generally includes a plurality of light-emitting elements, and a long and narrow strip-shaped board. The plurality of light-emitting elements are mounted on the board while disposed in series in a longitudinal direction of the board. Uniformly spacing the light-emitting elements on the board allows the linear light (i.e., the light in the longitudinal direction of the board) to have uniform intensity distribution (see PTL 1 and PTL 2). Thus, the light that enters the inside of the light guide plate has uniform intensity distribution. However, there arises a problem as follows. The board sometimes includes an identifying code that shows the type of the light source (i.e., a printed identification character). In addition, the board sometimes includes a screw hole arranged to fix the board to the illuminating device. In addition, the board sometimes includes a connector arrange to receive a power supply from the outside. Thus, the light-emitting elements need to be disposed so as to avoid the identifying code, the screw hole or the connector, so that the light-emitting elements on the board cannot be uniformly spaced, sometimes.
In addition, the light that leaks out of the light guide plate is reflected by a surface of the board to reenter the inside of the light guide plate. Because the identifying code on the board, the connector on the board, or a head of a screw arranged to fix the board sometimes have a lower reflectivity than the board surface, the light that is reflected by the identifying code, the connectors or the screw head to reenter the inside of the light guide plate is lower in intensity than the light reflected by the other portion of the board surface. For this reason, when a portion of lower reflectivity exists locally on the board surface, the light that enters the inside of the light guide plate becomes non-uniform in intensity.
Thus, the light that enters the inside of the light guide plate from a portion of the light guide plate, the portion opposing the identifying code, the screw head or the connector, is lower in intensity than the light that enters from the other portions. Consequently, intensity distribution in a plane of the light emitted from the front face of the light guide plat sometimes becomes non-uniform.